classdef spyfilt_diffy < spyview_filter
   properties (Constant=true)
       name='dZ/dy';
       description='Take a derivative along each vertical line';
       parameters=[];
   end
   methods
       function [pm,px,py]=filter(sf,pm,px,py)
           pm=diff(pm,[],1);
           py=(py(1:(end-1))+py(2:end))/2;
       end
   end
end